1. Field of the Invention
This invention relates to a plasma apparatus employed for fabrication of, for example, semiconductor devices, and a plasma processing method employing such plasma apparatus. More particularly, it relates to an apparatus allowing the gas dissociated state in a so-called high-density plasma, such as inductance-coupled plasma or helicon wave plasma to be controlled, and a method for carrying out plasma processing, such as dry etching, using such apparatus.
2. Description of the Related Art
As the integration degree of semiconductor devices proceeds to the stage of VLSIs and further to the stage of ULSIs, shrinkage in the minimum processing size is proceeding rapidly. For example, a design rule of 0.35 .mu.m is adopted in a 64 MDRAM scheduled to be mass-produced in the near future. On the laboratory level, studies on ultra-fine processing based on the design rule of 0.25 to 0.1 .mu.m are being conducted with a goal towards fabrication of semiconductor devices of the next and next-to-next generations.
The technique of dry etching consisting in etching a sample using ions or radicals in a plasma produced on discharge dissociation of an etching gas in a high vacuum chamber has contributed significantly to the progress in such fine processing technology. Among presently employed plasma generating systems, the electron cyclotron resonance (ECR) system and the magnetron RIE system are most prevalent.
It has however been found that certain limitations are met with the currently employed plasma if is desired to produce a fine pattern with the minimum processing size of 0.25 .mu.m or finer, as required in, for example, 256 MDRAMs.
For example, a strong magnetic field is employed in the above-mentioned ECR system or magnetron RIE system for increasing the plasma density. However, with the recent process employing a large gauge wafer 8 inches in diameter, it is difficult to produce a uniform magnetic field over the entire wafer surface. Thus the plasma density becomes nonuniform within the wafer plane, resulting in the tendency for gate insulating films to be destroyed. Another problem is that electrical charges accumulated on the substrate may produce an unusual etching shape due to differences in the capture ratio of electrons to ions with respect to the strong magnetic field effect in the ion incident direction.
Such a problem manifests itself by the generation of side etching to a W film 5e in the etching of an Al/W based laminated interconnect film, as shown for example in FIG. 1, in which the Al/W based laminated interconnect film on an interlayer insulating film 1 is etched using a patterned photoresist 9 and a fluorine-based gas. It is thus seen that, while the desired shape anisotropy is obtained with a Ti film 2a and a TiN film 3a, making up a barrier metal layer 4a, an Al-1% Si film 6a and a TiON anti-reflection film 7a, side-etching is produced on a W film 5e.
Under the above-mentioned ultra-fine design rule, intrinsic anisotropic etching, based on a clean process not resorting to sidewall protection, is desired. It is however necessary in such case to elongate the mean free path of ions by carrying out discharge under a low gas pressure.
Such low gas pressure discharge has an etching mechanism close to ion sputtering, and is crucial in the etching of a film of an SiO.sub.x based material for which sidewall protection is not intrinsically necessitated. The reason is that, with the coming into use of a multi-layer interconnection in a stacked capacitor in DRAMs, multi-layer polysilicon interconnection in a SRAM or a multi-layer interconnection in logic devices, a process of forming a connection hole having a high aspect ratio in a planarized thick SiO.sub.x based insulating film has made its debut.
However, under the low gas pressure, the chemical species in the plasma contributing to the etching are lowered in density thus raising problems such as a low etch rate or a low throughput. Consequently, for producing vanguard semiconductor devices, a demand is raised for a plasma apparatus in which the magnetic field can be lowered in the vicinity of a substrate and which is capable of generating a high-density plasma under a reduced gas pressure. Recently, a number of new types of high-density plasma apparatus have been proposed.
One of them is a helicon-wave plasma apparatus disclosed in JP Patent Kokai (Laid-Open) Publication No.3-68773 (1991). The plasma generating mechanism of the apparatus is that the helicon wave is generated in a cylindrical chamber by generating a magnetic field in the chamber and by applying the high frequency to a loop antenna wound about the chamber and the energy is transported from the helicon wave to electrons through the process of Landau damping for accelerating the electrons and colliding them against gas particles for producing a high ionization ratio. It is possible with the helicon wave plasma apparatus to achieve the ion density on the order of 10.sup.11 to 10.sup.13 /cm.sup.3 (16 to 20 mA/cm.sup.2 in terms of the ion current density) under a low pressure on the order of 10.sup.-4 Torr. By the way, the ion density achievable with the ECR plasma apparatus is on the order of 10.sup.11 /cm.sup.3 (10 to 15 mA/cm.sup.2 in terms of the ion current density) at most, depending on the operating conditions employed.
In the JP Patent Kokai (laid-Open) Publication No.6-112166, there is disclosed a plasma apparatus generating an inductively coupled plasma (ICP). With the plasma apparatus, the high frequency power is supplied to a non-resonant multi-turn antenna placed about a quartz cylinder defining a plasma generating chamber and electrons are rotated in accordance with the magnetic field formed within the antenna for colliding the electrons against the gas molecules at a high probability. With such ICP apparatus, the ion density on the order of 10.sup.11 to 10.sup.12 /cm.sup.3 can be achieved.
With the above-mentioned helicon wave plasma processing apparatus or ICP apparatus, since a strong magnetic field such as is employed in the conventional ECR system or magnetron system is not required for plasma generation, the magnetic field in the vicinity of the substrate can be significantly diminished or practically reduced to zero. Thus the nonuniform plasma density, ion oscillations or oblique ion incidence under the effects of the magnetic field may be significantly reduced for suppressing destruction of gate insulating films or unusual etching shape. On the other hand, since there is no necessity of employing a costly magnetic coil or a large-sized micro-wave generating apparatus, and an extremely high frequency need not be employed, the apparatus may be simplified and reduced in size and costs. This means a highly meritorious feature in consideration that the future tendency is towards the device fabricating apparatus having a multi-chamber construction.
However, there is presented such a problem that dissociation of gas molecules proceeds excessively in a low-pressure high-density plasma, such as ICP or a helicon-wave plasma, because of the extremely high electron temperature, as a result of which ions are generated in an excessive amount while radicals are generated in an insufficient amount.
In general, the high etch rate in dry etching is achieved in many cases by a so-called ion-assist mechanism in which the chemical reaction induced by radicals adsorbed on a sample surface is accelerated by the physical energy brought about by ion impact. Typical of such cases is the above-mentioned etching of the Al/W laminated interconnection layer. Consequently, in a system suffering from shortage in radicals, radical adsorption and ion irradiation are not repeated smoothly, thus lowering the etch rate.
In case of a system where ion sputtering plays the main part in the etching mechanism, relative shortage of certain radicals occasionally lead to lowered underlying layer selectivity. For example, when c-C.sub.4 F.sub.8 (octafluoro cyclo butane), a well-known etching gas for a layer of an SiO.sub.x based material, is employed, CF.sub.2 *, a precursor of a carbonaceous polymer contributing to Si selectivity, is generated in the conventional magnetron RIE device or ECR etching device, along with ions, such as CF.sub.2.sup.+ or CF.sub.3.sup.+, main etching species, for facilitating the progress of high speed high selectivity etching. However, within the ICP device or helicon wave plasma processing apparatus, dissociation of the etching gas proceeds excessively, such that monoatomic active species, such as C.sup.+ or F*, are generated in larger quantities, while polyatomic ions or polyatomic radicals, such as those given above, are drastically reduced in quantities. The result is that the polymerization reaction of carbonaceous polymers is not induced to a sufficient extent such that Si selectivity is lowered.
In order to solve this problem, it is contemplated to add depositive gases, such as CH.sub.2 F.sub.2 or CHF.sub.3, to the etching gas system. This method resides in capturing excess F* by taking advantage of H* released from the depositive gases to raise the C/F ratio (ratio of the number of C atoms to that of F atoms) in the etching gas system for accelerating deposition of the carbonaceous polymer. However, for achieving a practically sufficient selectivity, an excess amount of the depositive gas needs to be added, which in turn tends to deteriorate the particle level or reproducibility.
As another method, the above-mentioned JP Patent Kokai (Laid-Open) Publication 6-112166 shows an ICP device in which a plate of electrically conductive silicon (Si) containing impurities at a higher concentration is employed as a member equivalent to an upper lid of a high-vacuum vessel. The Si plate captures a part of fluorine radicals (F*) generated in larger quantities in a plasma on its surface and discharges the captured radicals out of the high-vacuum vessel as SiF.sub.x for improving the Si selectivity. The Si plate is rendered electrically conductive so as to be utilized as a large-area dc grounding electrode for the plasma as well.
However, since the Si plate is significantly lower in electrical conductivity than metal, it is necessarily raised in resistance such that there is a risk that the substrate bias cannot be not applied effectively.
As a further method, there has been proposed in Extended Abstract to the 40th Lecture Meeting of the Society of Applied Physics, Spring Meeting of 1993, page 529, lecture number 29p-ZE-8, a method consisting in controlling gas dissociation using a pulse-modulated ECP plasma. More specifically, this method resides in temporally controlling electron multiplication and loss using the pulse-modulated micro-wave for varying the plasma electron temperature, thus suggesting the possibility of high precision etching. However, the ECR plasma apparatus is in need of large-sized costly equipment, such as solenoid coils or a micro-wave source, and hence is inferior to the helicon wave plasma apparatus in economic profitability or space efficiency in clean rooms.